In open-heart surgery, blood which normally would pass through the heart, is by-passed in a circuit which includes a venous return cannula placed in the heart through an incision in the right atrial appendage. Blood is allowed to drain from the heart to a venous blood reservoir. Differential pressure to insure drainage can be provided by locating the reservoir lower than the heart. In that case, drainage is dependent on the differential created by the position of the heart and the reservoir. A preferred method of insuring and accelerating drainage is to utilize negative pressure or vacuum pressure. By employing vacuum, it is possible to increase blood flow and therefore reduce the internal diameter of the cannula. The reduction in diameter and increase in vacuum pressure differential is limited by the openings in the distal end of the cannula in a single-stage cannula and in the case of a two-stage cannulae, in openings in the distal end and also at an intermediate point spaced slightly from the distal end.
During surgery the openings or intake ports in the cannula must remain completely open without obstruction by tissue or organs coming in contact with the outer surfaces of the cannula to block the intake ports and the flow of blood. The possibility of this occurring increases with an increase in vacuum pressure differential. At the same time, the possibility of decreasing the diameter of the cannula and increasing blood flow is enhanced with an increase in the vacuum pressure differential that might be used.